1. No Time to Countdown: Backing Off in Frequency Domain
Souvik Sen,
Romit Roy Choudhury, Srihari Nelakuditi

2. Current WiFi Channel Contention
Random Backoff = 15
Random Backoff = 25
AP1
AP2 R1 R2
B1=15
AP1
DIFS
AP2
Time
B2=25 2

3. Current WiFi Channel Contention
Random Backoff = 0
Random Backoff = 10
AP1
AP2 R1 R2
B1=15
B1=0
AP1
DIFS
AP2
Time
B2=25
B2=10 3

4. Current WiFi Channel Contention
Carrier Busy
Transmit
AP1
AP2 R1 R2
B1=15
B1=0
AP1
Data and ACK
DIFS
AP2
Wait
Time
B2=25
B2=10 4

5. Current WiFi Channel Contention
Random Backoff = 18
Random Backoff = 10
AP1
AP2 R1 R2
B1=15
B1=0 B1=18
AP1
Data and ACK
DIFS
B2=10
AP2
Wait
Time
B2=25
B2=10 5

6. Current WiFi Channel Contention
Random Backoff = 8
Random Backoff = 0
AP1
AP2 R1 R2
B1=15
B1=0 B1=18
B1=8
AP1
Data and ACK
DIFS
B2=10
B2=0
AP2
Wait
Time
B2=25
B2=10 6

7. Current WiFi Channel Contention
Carrier Busy
Transmit
AP1
AP2 R1 R2
B1=15
B1=0 B1=18
B1=8
AP1
Data and ACK
Wait
DIFS
B2=10
B2=0
AP2
Wait
Data and ACK
Time
B2=25
B2=10 7

8. Current WiFi Channel Contention
Carrier
AP1 Busy AP2 Transmit
R1 R2
B1=15 B1=0 B1=18 B1=8
High channel wastage due to backoff
35% overhead at 54Mbps
AP1
Data and ACK
Wait
DIFS
B2=10
B2=0
AP2
Wait
Data and ACK
Time

9. Current WiFi Channel Contention
 Backoff is not fundamentally a time domain operation
 Its implementation is in time domain
Time Domain
Frequency Domain
Can we implement backoff in frequency domain?
Are there any benefits in doing so?

10. Frequency domain contention resolution
a/g/n PHY adopts OFDM
Wideband channel divided into 48 narrowband subcarriers
Copes better with fast, frequency selective fading
Purely a PHY motivation
Subcarriers: … 48
Frequency
We propose Back2F
MAC Opportunity: Pretend OFDM subcarriers as integers
Emulate randomized backoff

11.  Replace temporal with subcarrier transmission
Back2F: Main Idea
 Replace temporal with subcarrier transmission 6 18 47 47
AP1
AP2
Backoff = 6
Backoff = 18 R1 R2

12.  Replace temporal with subcarrier transmission
Back2F: Main Idea
 Replace temporal with subcarrier transmission
Other’s Backoff = 18
Other’s Backoff = 6 6 18 47 47
Listen Antenna
Listen Antenna
AP1
AP2
Backoff = 6
Backoff = 18 R1 R2
Both APs learn AP1 is the winner

13. Back2F: Scheduled Transmission
Active subcarriers imply backoff chosen by other APs
Each AP knows its rank in the sequence
Enables back to back TDMA like transmission
Self Backoff
Other’s Backoff 12 47
Rank in TDMA: 3
AP1

14. frequency domain backoff?
Is there a benefit with
frequency domain backoff?
1500 bytes at 54Mbps ~ 250 micro sec.
Avg. temporal backoff ~ 100 micro sec.
Big overhead
Frequency backoff = 1 OFDM symbol = 4 micro sec
4 microsecond overhead  dramatic performance enhancement relative to 100 microsecond overhead

15. Will APs Collide During Contention?
 Introduce a second round of contention
 Winners of first go to second
Subcarrier 1 2 3
First Round 4 5

16. Will APs Collide During Contention?
 Introduce a second round of contention
 Winners of first go to second
Subcarrier 1 2 3
First Round 4 5
Subcarrier 1 2 3
Second Round 4 5

17. Only a Few APs in Second Round?
TDMA will not be effective

18.  Instead of only winners, a few more APs to second round
Optimize for TDMA
 Instead of only winners, a few more APs to second round
Subcarrier 1 2 3
First Round 4 5
Subcarrier 1 2 3
Second Round 4 5

19.  Instead of only winners, a few more APs to second round
Optimize for TDMA
 Instead of only winners, a few more APs to second round 1 2 3
First Round 4 5
0 2 4
0 2 4
0 2 4
Enabling TDMA
Rank 1
Rank 2
Rank 3
Subcarrier 1 2 3
Second Round 4 5

20. Improved Channel Utilization
0 2 4
0 2 4
0 2 4
Enabling TDMA
Rank 1
Rank 2
Rank 3
Subcarrier 1 2 3 4 5

21. Improved Channel Utilization
0 2 4
0 2 4
0 2 4
Enabling TDMA
Rank 1
Rank 2
Rank 3
Subcarrier 1 2 3 4 5
Data/ACK
Data/ACK
Data/ACK
Time
802.11: Contention per packet
Frequency Backoff
TDMA
Data/ACK
Time
Back2F: Contention per TDMA Schedule

22. Multiple Collision Domains
 Does Back2F work with real-world scattered APs?

23. Multiple Collision Domains
 Does Back2F work with real-world scattered APs?
BO = 5
BO = 2
BO = 3
BO = 1
Blue waits for Purple, but Purple waits for Green
But Blue and Green should transmit simultaneously
 Lost transmission opportunity 
However does not suffer from this problem
Blue will wait for DIFS, continue counting down and eventually transmit

24. Multiple Collision Domains
 Does Back2F work with real-world scattered APs?
BO = 5
BO = 2
BO = 3
BO = 1
 Blue waits for Purple, but Purple waits for Green 
But Blue and Green should transmit simultaneously
 Lost transmission opportunity
Back2F Solution: Emulate 
However does not suffer from this problem
Blue will wait for DIFS, continue counting down and eventually transmit

25. Multiple Collision Domains
BO = 5
BO = 2
BO = 3
BO = 1
Frequency Backoff
G -> 1
R -> 5 4
P -> 2
DIFS
Time
B -> 3 2
DIFS
Reduce BO
by winner’s BO
Channel idle > DIFS
Frequency Backoff, Wait for turn
Frequency Backoff
My turn
Transmit

26. Back2F: Performance Evaluation
Three important questions:
Can Back2F detect subcarriers reliably?
What is Back2F’s collision probability?
How much throughput gain over ?

27. Back2F: Performance Evaluation
 Three important questions:
 Can Back2F detect subcarriers reliably?
 Evaluated on USRP/Gnuradio
What is Back2F’s collision probability?
How much throughput gain over ?
 Evaluated using traces at 65 locations

28. Back2F: Performance Evaluation
 Three important questions:
 Can Back2F detect subcarriers reliably?
 Evaluated on USRP/Gnuradio
Practical Challenge: High Self Signal
Transmit Antenna
Listen Antenna

29. Self Signal Overflows into Adjacent Subcarrier60 50 40 B
n 30 NR 20 10 60
64 pt. FFT
64 pt. FFT 50
Self Signal 40 30 20 10
S SNR in dB d i 10
Subcarrier Number 20
30 40
Subcarrier Number 50 60

30. Solution: Use a Higher Point FFT60 60 50 40 B
n 30 NR 20 10
128 pt. FFT
128 pt. FFT 50
Self Signal 40 30 20 10
S SNR in dB d i 10
Subcarrier Number 20
30 40
Subcarrier Number 50 60

31. Solution: Use a Higher Point FFT60 60 50 40 B n 30 NR 20 10
256 pt. FFT
256 pt. FFT 50
Self Signal 40 30 20 10
S SNR in dB d i 10
Subcarrier Number 20
30 40
Subcarrier Number 50 60

32. Subcarrier Detection Performance1
0.8
0.6
0.4
0.2
8dB SNR
10dB SNR
12dB SNR
14dB SNR
Detection Accuracy
0 1
Distance in Subcarriers
Robust subcarrier detection at 14dB
Reliable subcarrier detection at 14dB

33. Back2F: Performance Evaluation
Collect traces to answer:
What is Back2F’s collision probability?
How much throughput gain over AP
Client
20 AP locations
45 client locations

34. Back2F: Performance Evaluation
Collect traces to answer:
What is Back2F’s collision probability?
How much throughput gain over
RSSI
Per Subcarrier SNR
Optimal Bitrate
Traffic pattern
Collision Probability
Emulate , Back2F
for various topologies

35. Back2F: Collision Probability
0.7
Back2F: 2 rounds Back2F: single round
802.11
Collision Probability(%)
0.6
0.5
Benefit of second round
0.4
0.3
0.2
0.1
Number of APs
Small collision probability in dense networks

36. Throughput Evaluation1
0.8
0.6
0.4
0.2
HD streaming
Skype traffic Web browsing
CDF
Throughput gain over
Higher throughput gain for real time traffic 36

37. Throughput Evaluation
Throughput gain (%) over
0.6
0.5
0.4
0.3
0.2
0.1
Number of clients
6 Mbps
8 Mbps
4 Mbps
/o batch 1 3 5
54 Mbps w
Throughput Gain of Upto 50%

38. Limitation and Discussion
Robustness of subcarrier based backoff
Back2F more sensitive to channel fluctuation
Need for additional antenna
Back2F is complementary to MIMO
Gain over packet aggregation
Aggregation may not be possible for real time traffic
Back2F provides gain with aggregation at higher rates
Interoperability with
May interoperate but will cause unfairness

39. Summary  Using OFDM subcarriers
Randomization is an effective method for contention resolution
time domain backoff requires channel to remain idle
Observation: randomization possible in frequency domain
 Using OFDM subcarriers
Back2F: practical system realizing frequency domain contention
Prevents collisions, provides upto 50% improvement in throughput

40. Duke SyNRG Research Group
Questions, comments?
Thank you
Duke SyNRG Research Group